### Motivation
Expose metrics in Prometheus regarding the number of log messages printed by
each process.
This will add metrics like:
```
# TYPE log4j2_appender_total counter
log4j2_appender_total{cluster="standalone",level="debug"} 0.0
log4j2_appender_total{cluster="standalone",level="warn"} 1.0
log4j2_appender_total{cluster="standalone",level="trace"} 0.0
log4j2_appender_total{cluster="standalone",level="error"} 0.0
log4j2_appender_total{cluster="standalone",level="fatal"} 0.0
log4j2_appender_total{cluster="standalone",level="info"} 181.0
```
[ Full content available at: https://github.com/apache/pulsar/pull/2735 ]
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